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Mathematical relationships have been developed for reaeration rate coefficient (Ka) by various researchers. These relationships have a number of variables such as depth, velocity, width, slope, Froud number, molecular diffusion coefficient, kinematic viscosity and the gas‐transfer Reynolds number. From these variables, 29 relations have been developed and divided into four groups. To evaluate their predictive capability for highly variable flow rivers receiving high pollution loads form large cities, these relationships have been used to model dissolved oxygen (DO) in the River Ravi. Such rivers are either saturated with DO during high flows or anaerobic during critical low‐flow conditions. The evaluation is based on the agreement between model DO values calculated using Ka obtained from the available equations and the measured DO concentrations in the river samples in terms of sum of square of residuals (SSR) and coefficient of determination (R2). It has been found that in general, the group of equations containing depth and velocity as the only two variables affecting Ka performed better than the equations in other groups as reflected by lower SSR and higher R2 values. The study results also reveal that the turbulence‐based reaeration rate coefficient equation containing additional variables also resulted in close agreement between DO model results and the measured values. The study results identify the most important parameters affecting the reaeration rate coefficient and the suitability of various Ka relationships as well for rivers with highly variable flows. Copyright © 2012 John Wiley & Sons, Ltd.

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Studies were conducted to evaluate the efficiency of cationic polymers as a suitable replacement of metal salts for the treatment of a local tannery wastewater. Eleven cationic polymers of varying molecular weights (MW) and charge densities (CD) were examined using jar test apparatus. Three cationic polymers: one with MW of 4 million Dalton and CD of 55%; second with MW of 6 million Dalton and CD of 40%; and the third with MW of 8 million Dalton and CD of 40% were found suitable for tannery wastewater treatment at an optimum dose of 20 mg/L for each. Percentage removals with these three cationic polymers for turbidity, total suspended solids (TSS), total chemical oxygen demand (TCOD) and chromium lied in a range of 91-95%, 69-83%, 25-29% and 96-97%, respectively with respect to plain settled wastewater. The cost of the most suitable cationic polymer C-496 at its optimum dose was $0.09 per cubic metre of wastewater and sludge production was 35 mL/L. The results demonstrated that treatment of tannery wastewater with cationic polymers is a viable and economical option when compared with metal salts.

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